1. Field of the Invention
The present invention relates to a rotation information detecting apparatus which generates a periodic changing magnetic field according to the rotation of a rotation object and detects rotation information of the rotation object on the basis of the periodic changing magnetic field. The rotation object is, for example, an internal combustion engine and the invention is applied to a use for generating rotation angle information of this internal combustion engine.
2. Description of the Related Art
For example, JP-A-11-304416 (called a conventional art) discloses a first kind of detecting apparatus in which a magnetic resistance element (called an MR element) is a detection element. This MR element is such that its electric resistance value is changed according to a change in direction of magnetic flux intersecting with a current direction in a ferromagnetic thin film, and the direction change of the magnetic flux to the ferromagnetic thin film is given according to the rotation of a rotation object, so that a detection signal in response to the direction change of the magnetic flux is generated.
Besides, the conventional art discloses a second kind of detecting apparatus in which a hall element is a detection element. This hall element generates a detection signal according to an intensity change of magnetic flux given thereto.
Each of the first and the second kind of detecting apparatuses disclosed in the conventional art includes a signal processing circuit for converting the detection signal generated in the detection element into a binary output signal changing between a high level and a low level. The signal processing circuit includes an output transistor for generating the binary output signal, the output transistor is connected to an output line, and the output signal is outputted in this output line. The output signal changes between the high level and the low level in response to the periodic changing magnetic field between a concave part and a convex part formed on the peripheral surface of a rotation plate rotating together with the rotating object to be detected. In a state where the periodic changing magnetic field does not change, the output signal keeps the high level or the low level.
However, in the first and the second kind of detecting apparatuses disclosed in the conventional art, there is a problem that when noise is superimposed on the output line at the time when the output line keeps the high level, the potential of the output line is erroneously changed from the high level to the low level by this noise. This erroneous potential change of the output line gives an error to detected rotation information, and causes a disadvantage that the erroneous rotation information is detected.